Backpack



FIG. 1 is a front perspective view of a backpack, showing the backpack with a front windowpane, in accordance with the present invention;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines are to disclose the environmental elements related to the claimed design, which form no part of the claimed design. 

CLAIM The ornamental design for a backpack, as shown and described. 